The well-known Manchester coding technique uses a bi-phase code, in which each transmitted bit is encoded as two bits, represented by an "on" state and an "off" state. Because of its high energy content at the clock recovery frequency, Manchester coding is advantageous in many applications such as those where data scramblers are not feasible or in connection with very long transmission systems with large numbers of repeaters.
To regenerate Manchester code in a timing recovery circuit requires the determination of which of the Manchester data signal transitions are the "bit-center" and which transitions present are "bit-edge" transitions. This uncertainty is called "phase-ambiguity," since normal NRZ Clock Recovery techniques will not resolve the proper phase.
Prior art schemes for making this determination exhibit some disadvantages. Among these is that a check for proper phase must be generated at twice the data frequency. Further, in the prior art schemes, output clock jitter will be pattern-dependent and will disadvantageously add to the signal being processed as a correlated source. Additionally, later error detection and fault location in the converted data stream are made more difficult.